This invention relates to power units actuated hydraulically and more particularly to hydraulically actuated fluid pumps. This apparatus is especially useful in pumping fluids from wells and in particular, in the pumping of fluids from relatively deep wells.
The majority of well pumps include a pump rod that goes down into the well and is connected at the upper end to a polish or sucker rod and includes a pump string, all of which is reciprocated vertically by suitable apparatus in order to pump the liquid from the well. At the bottom end of the pump rod string, a pump is provided including a plunger, foot valve and the necessary packing glands, etc. Pumps of this character are well known and are not shown or described in the present application.
Wells can be relatively shallow or relatively deep, frequently extending downwardly to a depth of 5,000 feet or more and the deeper the well, the greater the number of problems that are present.
These problems also increase in severity as the depth of the well increases. One of the difficulties encountered is that when the rod is at the bottom of the stroke, the weight of the rod, the weight of the oil to be lifted during the upward stroke of the rod and the frictional resistance to movement provided by the pump parts, combine to offer considerable inertia or resistance to upward movement that must be overcome by the power apparatus that reciprocates the rod so that under normal circumstances, relatively high horsepower sources of power are required.
Furthermore, in conventional pumps, in extremely viscous liquids, the full lifting force is instantaneously shifted from a downward direction to an upward direction when the rod cannot keep pace with the descent of the lifting apparatus and the magnitude of this lifting force and its instantaneous application sometimes causes an elongation of the pump rod string and in time often causes considerable wear to occur or causes parts of the pump to tear loose.
Further, in actual practice it sometimes happens that the rod string in its downward movement causes the pump plunger to impact upon the bottom of the well with sometimes injurious results to the entire apparatus as well as to the foot valve within the pump at the bottom of the well.
It has therefore been necessary to reduce the reciprocatory velocity of the pump rod string so that a greater length of time is expended in changing the direction of movement of the rod string at the upper and lower ends of its stroke. This approach is disadvantageous for at least two reasons, one being that a greater amount of time must be spent in removing a given volume of oil or fluid from a well, particularly where the viscosity of the fluid is very high and a very slow pumping cycle is present, and the other being that there is a certain amount of leakage in the foot valve which is a function of time so that if a greater time is expended in lifting the oil or other fluid, a greater amount of this oil or other fluid will be lost through leakage.